I. Field of the Invention
The present invention relates to the field of acoustic transducers. More specifically, the present invention relates to a novel electrostatic ultrasonic transducer capable of operating in high frequency ranges, and novel methods of fabricating such a transducer.
II. Description of the Related Art
An acoustic transducer is an electronic device used to emit and receive sound waves. An ultrasonic transducer is a type of acoustic transducer that operates at a frequency range beyond that of human perception, about 20 KHz Acoustic transducers are used in medical imaging, non-destructive evaluation, and other applications. The most common forms of acoustic transducers are piezoelectric transducers, which operate in low and narrow band frequencies. Piezoelectric transducers are not efficient in the conversion between electric and acoustic energy in air. Furthermore, the operating frequencies of piezoelectric transducers in air are quite low.
Air coupled ultrasonic transducers with higher operating frequencies, which rely on certain microfabrication techniques, are described by Haller et al. in U.S. Pat. No. 5,619,476 entitled "Electrostatic Ultrasonic Transducer," issued Apr. 9, 1997, and Ladabaum et al. in U.S. Pat. No. 5,870,351 entitled "Broadband Microfabricated Ultrasonic Transducer and Method of Fabrication," issued Feb. 9, 1999. Published material known in the art also demonstrates that immersion transducers can be made with similar techniques. Air-coupled transducers are usually resonant, while liquid-coupled transducers are typically not. As shown in FIGS. 1A and 1B taken from the '476 patent, the transducer disclosed therein is made of a substrate 11 and a gold contact layer 14 that forms one one plate of a capacitor, and a membrane including a nitride layer 13 and a gold contact layer 14B that form the other plate of the capacitor (while the gold contact layer 14 is the electrode, with the nitride layer 13 being an insulator, the reference to electrode 13/14 will be used so as to distinguish the other electrode 11/14 that has a gold contact layer 14 adjacent the conductive substrate 11 as illustrated in the above-mentioned patents). Holes 16 etched in the nitride layer 13 and the gold layer 14 are used to etch away portions of the sacrificial oxide layer 12, while remaining posts of the sacrificial layer 12 support the membrane. By noting the change in capacitance between the two electrodes 13/14 and 11/14, the ultrasonic resonance of the membrane can be detected.
Such microfabricated ultrasonic transducers use resilient membranes that have very little inertia. The momentum carried by approximately half of a wavelength of air molecules is able to set the membrane in motion and visa versa. Electrostatic actuation and detection enable the realization and control of such resonant membranes. When distances are small, electrostatic attractions can exert very large forces on the actuators of interest.
Microfabricated ultrasonic transducers of this design have practical problems that prohibit their use at high frequencies, typically above about 10 MHz, and that reduce their efficiency at any frequency range. It has been realized by the present inventor that there are various reasons that prohibit the use of microfabricated ultrasonic transducers. One reason is that the electrodes 13/14 and 11/14 are each formed as a conductive sheet. As illustrated in FIG. 1A, while the gold contact layer 14 covers the voids where the sacrificial layer 12 has been etched away, the gold contact layer 14 also entirely covers the posts which support the membrane. Similarly, the substrate 11 and the gold contact layer 14 associated therewith is another conductive sheet. Accordingly, at areas other than where sacrificial etch access holes 15 exist, there is no area where the electrodes 13/14 and 11/14 do not overlap. This overlap causes a parasitic capacitance, which is exacerbated due to the fact that the dielectric constant of the semiconductor insulators between the areas of the sacrificial layer 12 posts can be approximately one order of magnitude larger than that of the air/vacuum gap at the center of the membrane. As frequencies become higher, the parasitic capacitance becomes significant and sometimes even a dominant factor in transducer performance. Thus, even if the overlap at the areas of the sacrificial layer 12 posts accounts for only 1/10 of the active area of the transducer, such overlap may account for half the capacitance.
Furthermore, the spacing between the top electrode 13/14 and the bottom electrode 11/14 is a further reason that the parasitic capacitance increases. In particular, the membrane has a thickness that, due to physical constraints, needs to be at least about 2,500 Angstroms thick. Thus, when the gold contact layer 14 is placed over the nitride layer 13, there is additional parasitic capacitance due to the thickness of the nitride layer.
As a result of the above-mentioned parasitic capacitances, transducers such as those described in Haller et al or Ladabaum et al are not able to operate at higher frequencies, and operate less efficiently than ultimately possible at lower frequencies. Accordingly, there is the need for an improved acoustic transducer.